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CEMF clamp diodes — Parallax Forums

CEMF clamp diodes

FlyingFishFingerFlyingFishFinger Posts: 461
edited 2006-04-15 23:24 in General Discussion
Hi!
Motor control trouble again....So, I have a 10A relay-and-mosfet controller. I am worried about the CEMF. What I have now is something very weird, advised by a guy at Gateway Electronics, who probably misunderstood me. He gave me a 1N4003 or something to that effect and told me to put it in line with my load...which makes no sense, because I'm running PWM through it as well. I also have some coils in line to absorb stuff, but I'm still worried. Also, I consequently have the~1V voltage drop in there. Luckily (I believe) I have not stuffed 10A through it yet, still testing...
Any advice?
Rafael Send

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-04-15 04:36
    f-f-f,
    · Do you have a scanner?· Maybe you could U/L a schematic of your circuit?

    N.B. --
    · Typically, you want to have your "CEMF" diodes across/in parallel with·your relay coils (reverse-biased).· If you're doing a H-bridge, then you (also) want to have "CEMF" diodes across your semiconductors (in parallel with C-E, S-D -- again, reverse-biased.)

    Post Edited (PJ Allen) : 4/15/2006 2:44:36 PM GMT
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 20:20
    Hi!
    So, I don't have a scanner, so I will try drawing it in software.
    I have the diode across my relay coil, I was wondering about the motor emf...so for that I put a diode across the mosfet?
    Rafael
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 21:35
    Ok here we go...probably the most messed-up design you ever saw. Note the transistor # and its normal application (I should replace it), Then the relay are actually two SPDT rated 9V but I'm runnung them on 5V, which works somehow. Last, notice the square boxes on the motor leads...they're those coils that look like those small transformers and are supposed to be absorbing spikes, according to a guy at Gateway...
    However, the whole setup works at low current...I haven't tried to stuff 10A through yet, luckily...
    Rafael
    *&#@^$&*Attachements(@#&@*()% burger.gif

    Post Edited (flyingfishfinger) : 4/15/2006 9:59:35 PM GMT
    circuit.JPG
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  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-04-15 22:17
    Are you using the Relay/s to change the direction of the Motor?· Here's how to do that.

    A 9v relay coil may energize with as little as 5V (not so surprising.)· I think you should turn the relay on by sinking through the transistor [noparse][[/noparse]also shown.]

    I wouldn't use the coils in series with the motor (a-la "Gateway".)

    Schematic attached.

    -- PJA --
    fff_relay+.jpg
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  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 22:23
    Ok. I get that, but you changed the mosfet setup. Why can one not connect the motor between ground and the mosfet?
    Also, what sort of diode should I use on the mosfet? Yes, it is PWM and remember it's a 10A motor at 12V on there...
    You said "also shown" - do you mean your schematic or mine?
    Thanks
    Rafael

    Post Edited (flyingfishfinger) : 4/15/2006 10:29:46 PM GMT
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-04-15 22:31
    · You can place a Motor between Source and Ground, that's a "source follower" configuration.· It's like an emitter follower, the voltage across the motor will be approximately = VG.· When sinking through the transistor/FET, the voltage across the Motor·is independent of VG (it is basically +V, supply voltage.)
    · Here's much the same but with an added CEMF diode -- it's more important than the other across the FET (it's seen next to the top contacts.)
    · I would use 1N4007.
    ffff_relay2.jpg
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  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 22:34
    What sort of diodes are these? Surely not 1Nsomethings?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-04-15 22:36
    Aye, surely they are... 1N4007.

    Y'know the current of the motor has nothing to do with the diode ratings.· As the diodes are backward-biased, the motor current does not pass through them.· They are serving as a current path for short-lived kick-back spikes (which aren't a function of or proportional to·motor current.)
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 22:39
    Am I safe with a 1N4003?
    My Robot DNA book puts a diode from the mosfet-relay connection to V+...???

    Post Edited (flyingfishfinger) : 4/15/2006 10:45:05 PM GMT
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-04-15 22:41
    What are they rated at?· 1N4007s are 1kV (PIV), that's why I "chose" them,·that's rating a-plenty.· You want Insurance?· This is·Assurance.
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 22:45
    Well they're rated 200V [noparse]:)[/noparse]
    Not good enough I guess
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-04-15 22:49
    · It's a tough call.· I figure you can't go wrong in this application with a 1kV diode, especially when they all (1N-series) cost the same.

    · What's the worst that could happen?· The $2 FET might go POOF!· End of The World?
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 22:53
    Naa...guess not..anyway, I have some rewiring to do...all the FET and transistor to source-followers...\
    Thanks and forgive my inexperience in this matter...it's my first current controller [noparse]:)[/noparse]
    Rafael
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-04-15 22:55
    Here's a last re-rendering of the FET and CEMF.

    It's the same as my second (previous), only shown more clearly.


    Post Edited (PJ Allen) : 5/28/2006 5:57:55 PM GMT
    relays_polswitch.JPG
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  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 22:56
    Ok
    Thanks a bunch
    Rafael
  • FlyingFishFingerFlyingFishFinger Posts: 461
    edited 2006-04-15 23:09
    Oh - can those diodes handle the the PWM frequencies?
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-04-15 23:24
    The PWM freq isn't a factor for the "CEMF" diodes.
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